In conventional photomask manufacturing processes, after a mask is written, developed and cleaned, it is normally inspected on a defect inspection machine and then, if defects are found, reviewed or repaired on a separate machine. Using conventional systems, the information of the photomask inspection such as the defect type and coordinates of the defect can be automatically transferred to the defect repair or review systems. Currently, there are three types of repair techniques: focused ion beam (FIB), far field laser repair which uses optical lenses, and the near field optical repair which uses a micro pipette to deliver the laser beam. Review of photomask defects is normally performed using an optical microscope.
The resolution and accuracy of the x-y translation stage on both inspection and repair machines individually can be as accurate as 5 nm to 10 nm. However, due to the possibility that the origin of the coordinate system encompassing an x-y translation stage is not recorded accurately, calibration between any pair of inspection and repair machines could be off by as much as 40 microns. As a result, using the location coordinates recorded on one machine to find the same defect or other particularized point on another machine may not in itself be enough to locate the point on the other machine. In some cases the defects found on the first machine can not be located by using the first machine's coordinates on the second machine, even when using the maximum field of view.
According to standard procedures, each repair technique has a distinct method to locate defects found on an inspection machine. For example, the FIB repair machine uses the Ga+ ion beam to scan the mask surface, while the near field optical system uses an AFM to capture the image. The far field laser repair machine will use a lower magnification objective if it is necessary. Several problems arise when defects fall outside the field of view on the repair or review machine. In the case of an FIB repair machine, the Ga+ beam could unnecessarily damage a large area of the mask surface. In addition, the repair/review process is more time consuming on both the FIB and the AFM machines when the operator has to scan for the defects instead of driving directly to the location. Certain types of defects, such as the clear extension or the Cr extension, with size less than 0.2 microns, can not be easily recognized with a low magnification microscope objective such as that used on a far field laser repair machine.
In many cases, the mask will be reviewed on an optical microscope after inspection in order to classify the defect type or to study the detail structure of the defects. Similar to the far field laser repair machine, it is normally very time consuming to find small defects which fall outside of the field of view. What is needed is a more accurate and efficient way to find defects with a mask repair or review device.